Growth factors play an important role in cell specialization and are promising factors for controlling stem cell differentiation or reactivating dormant biological processes in vivo, both of which may lead to the regeneration of nonfunctional tissues. Growth factors are quickly degraded and rapidly diffuse away from a cellular site or injury. Accordingly, an architecture or scaffold would be useful to retain the growth factor at such a site for release to the surrounding cells in a controlled fashion.
Materials designed molecularly for tissue regeneration are becoming of great interest in advanced medicine. Scaffolds of synthetic polymers, including polymers based on L-lactic or glycolic acid, and biopolymers including collagen, fibrin or alginate have been studied. Growth factors have been physically entrapped in hydrogels as such polymers, covalently linked or bound electrostatically to either anionic polymers or structures such as heparin. Drawbacks to these and related systems relate either to non-specificity of bound growth factors or required degradation of covalent bonds to achieve desired effect. More recently, both natural and synthetic scaffolds have been modified to contain peptides found in extracellular proteins that promote receptor based interactions with cells and have been used to promote cell adhesion or differentiation.
Advances in self-assembly offer new opportunities in molecular design of biomaterials. Amphiphilic molecular building blocks can be assembled in aqueous environments to form scaffolds with well defined and diverse chemical structures. Various classes of peptide-based amphiphiles have been reported in the literature, including amphiphilic peptides and peptides functionalized with hydrophobic components (e.g., alkyl tails) on one or both termini. Amphiphilic peptides have been shown to form a variety of supramolecular structures like nanotapes, ribbons, fibers, and twisted ribbons. These structures originate from β-sheet formation among the amphiphilic molecules. A special case is that of amphiphilic peptide block copolymers that form gels with properties strongly dependent on the secondary structure of the individual peptide blocks. An example of peptide amphiphiles with alkyl tails on one terminus include amphiphiles derived from peptide motifs found in collagen which result in the formation of triple helical units that form spheroidal or disc-like micellar structures depending on the tail length and number of tails. Another class has two tails, one per terminus. This class of amphiphiles displays amyloid-like behavior in that, upon increasing the concentration, they undergo a transition from a random coil to a β-sheet type conformation that leads to fibrillar structures. There are reports of purely peptidic nanostructures with antiparallel arrangements, and one report of two modified peptide amphiphiles that aggregate in an antiparallel arrangement driven by an unnatural alkylated quaternary ammonium salt.
A class of peptide amphiphiles (PAs) is disclosed in one or both of two co-pending applications comprising a linear hydrophobic tail coupled to a peptide block that includes β-sheet forming segments, charged residues for solubility, and biological epitopes. The alkyl tail is attached to the N-terminus of the peptide, and the epitope segment is placed at the C-terminus. Upon application of a trigger such as a change in pH or ion concentration, these PA molecules can self-assemble in an aqueous medium into nanofibers. The alkyl chains are in the core of the fibers, with the epitopes displayed on the periphery for cell interaction. Epitopes that have been incorporated into the PA molecules mimic extracellular matrix proteins and promote cell adhesion or differentiation through cell signaling. It has also been shown that two different PA molecules with different epitopes and complementary charge can be co-assembled into the same nanofiber. See, co-pending application Ser. No. 10/368,517 filed Feb. 18, 2003 (international publication number WO 03/070749) and application Ser. No. 10/294,114 filed Nov. 14, 2002 (international publication number WO 03/054146), each of which are incorporated herein by reference in their entirety.
However, such PA compounds are typically prepared via solid phase synthesis with the peptide component generated from the C-terminus to the N-terminus. Coupling a hydrophobic component to the N-terminus provides a PA compound with either a free acid or amide group on the periphery of the resulting micellar nanofiber assembly as various epitope or peptide sequences often require a free N-terminus for bioactivity, such PA compounds would be ineffective for growth factor interaction.